Field of the Invention
The present invention relates generally to the technology of induction heating and more particularly to the use of induction heating devices for case-hardening of machine components such as gears.
Machine components such as gears, splined shaves and sprockets are frequently subjected to high torque loads, frictional wear and impact loading. Gears of this type are typically used in power transmission drive trains. An apparatus and method for induction-hardening of such machine components is disclosed in U.S. Pat. No. 4,845,328 to Storm et al., the contents of which are hereinafter incorporated by reference. The Storm et al. patent and this application are both owned by the same assignee, Contour Hardening Inc., of Indianapolis, Ind.
As is well known in the art, a known device for gear teeth hardening includes a dual-frequency arrangement for induction heating wherein a low frequency current is used for preheating the gear teeth and then a high frequency (Radio Frequency) current is then used for final heating prior to quench hardening of the gear teeth. The dual frequency induction hardening concept is described in the article "Induction Gear Hardening by the Dual-Frequency Method" which appeared in Heat Treating Magazine, Vol. 19, No. 6, published in June, 1987.
As explained in the article, dual-frequency heating employs both high and low frequency heat sources. The gear is first induction heated with a relatively low frequency source (3-10 kHz), providing the energy required to preheat the mass of the gear teeth. This step is followed immediately by induction heating with a high-frequency source which typically ranges from 100-300 kHz depending on the gear size and diametral pitch of the gear teeth. The high-frequency source will rapidly final heat the entire tooth contour surface to a case hardening temperature. The gears are then quenched to a desired hardness and tempered.
Induction heating is the fastest known way of heating an iron alloy gear. In some applications a preheat low frequency heat process precedes the final heat RF heating. Heating times for the high-frequency RF heating step typically range from 0.10 to 2.0 seconds. In induction heating, the gear is mounted on a spindle and spun while positioned within the induction heating coil. A quick pulse of power is supplied to the induction heating coil which achieves an optimum final heat of the gear teeth. Next, the piece is manually or automatically moved into a water-based quench. Because induction hardening puts only the necessary amount of heat into the part, case depth requirements and distortion specifications are met with great accuracy.
Within the induction heating process, whether dual- or single frequency, and regardless of the type of part and its material, the part characteristics dictate the optimum design of both the induction heating coil or coils and the most appropriate machine settings. In particular, the amount of time that the high-frequency power signal is supplied to the induction heating coil to generate the final heat is a most critical parameter. The exact amount of heat required to harden the gear is directly related to the precise amount of time that the power signal is supplied to the induction heater coil.
Traditionally, there are two systems well-known in the art for supplying power to an induction heater coil as described above. The first system utilizes what is known in the art as a "solid state" generator approach wherein high power amplification devices such as transistors, be they bipolar or CMOS, are used in the high-frequency RF generator to supply a high-frequency oscillator signal to the induction heater coil. An alternate approach is to use a vacuum tube RF generator and utilize thyristor type devices to switch power on and off to the high-frequency, high power vacuum tube oscillator circuit. The output of either oscillator circuit is coupled to the induction heater coil by way of a transformer. Some experts in the art of induction heating coil machines designed for case hardening metallic structures have heretofore preferred the solid state high-frequency RF generators for their exact timed control of power delivery to the induction heater coil. A vacuum tube RF generator typically receives its input power subject to the on/off timing characteristics of thyristor devices such as silicon controlled rectifiers (SCR's) which are also known in their JEDEC description as reverse blocking triode thyristors. The power delivery timing variance created by the SCR is intrinsic in the operation of such devices. Specifically, once an SCR is "turned on" for a partial cycle, even though the on/off signal supplied to the gate is removed or deactivated, the SCR will continue to conduct current so long as the anode to cathode terminals are biased with a positive voltage. In the worst case of a 60-cycle power signal being transferred by the SCR, this results in over an 8 millisecond additional power signal transmitted by the SCR, since half of a 60-cycle waveform is 8.33 milliseconds in duration.
It is recognized that the vacuum tube RF generator is preferred by some in the induction heating art for its characteristic power delivery curve in supplying power to an induction heater coil. Additionally, since SCR's are the device of choice for repeated high power switching circuits, a technique for accurately controlling SCR's to deliver specific quantities of power to a high-power vacuum tube RF generator is needed.
A method and apparatus for more accurately controlling the timed power output of a silicon controlled rectifier power supply is needed for accurately controlling the power signal supplied to induction heater coils used in case hardening devices.